PARIS—Former French President Nicolas Sarkozy took to national airwaves Wednesday to express shock and defend himself hours after he was officially declared as a suspect in a corruption and influence-peddling investigation, jeopardizing his chances of a political comeback.

In an interview with French TV, Mr. Sarkozy denounced his treatment by investigating magistrates who took the unprecedented step Tuesday of detaining the former president in Nanterre, near Paris. After 15 hours of police questioning, the magistrates...